The present invention relates to a radio communications system using time multiplexed frequencies, in which system transmitted symbol sequences in time slots of the system have a transmission point in time which is adjusted at a transmitter. Through this adjustment of the time, the symbol sequences are received by a receiver at a predetermined point in time in the time slot.
For time-divided multiplex radio communication systems it is of great importance that transmitters and receivers are well synchronized. Different transmitters, transmitting to a common receiver utilizing a common carrier frequency, have to transmit their symbol sequences in a well-defined order in consecutive time slots and at predetermined times in the time slots. If the synchronization is deficient, the symbol sequences from the different transmitters will overlap and interfere with each other at the receiver. This problem is particularly apparent for land mobile radio systems. As the mobile radio stations of the system move in relation to a stationary or at least quasi-stationary base station, time shifts of the symbol sequences arriving at the base station appear. Even if a specific mobile and the base station are well synchronized at the beginning of a connection, the time shifts may cause loss of synchronization. If, for instance, the mobile moves away from the base station, the symbol sequences will be delayed and will impose on the next time slot, which belongs to a different mobile.
The above stated problem with deficient synchronization is well known, and various methods have been developed in order to retain the synchronization between two radio stations. Taken as an example, for the Japanese mobile telephone system a standard called RCR STD-27B has been elaborated, describing a method of successively adjusting the point in time of transmission in a time slot belonging to the mobile. The base station measures during the course of a call the point in time when the symbol sequences from the mobile arrive at the base station and compares this point in time with a predetermined point in time in the current time slot. If a delay of a full symbol time is detected, the base station will send an adjustment message ordering the mobile to advance its time of transmission by one symbol time in relation to the reference time of the mobile. The adjustment order is issued only once and no acknowledgment is sent by the mobile. The measurement of the time position continues and the base station detects once more a delay of a full symbol time. The base station issues a renewed adjustment order to the mobile to advance its time of transmission, this time by two symbol times in relation to the reference time, upon which the mobile performs the order. If, however, the mobile did not detect the first adjustment order and no further time shift has occurred, the base station will nevertheless measure a delay of one symbol time at the second measurement. According to the standard, the base station is allowed, on this occasion as well, to issue another adjustment order to the mobile to advance its time of transmission by two symbol times in relation to its reference time, as it is presupposed that the mobile has really received and performed the first order. If the mobile detects the second order, this order will anyhow be neglected since the standard also prescribes that deviations of only one single symbol time from the current time position may be performed after the adjustment order. The procedure continues by the base station measuring the delay to be once more one symbol time, ordering the mobile to adjust by three symbol times in relation to its reference. This way, the synchronization may be lost and the mobile may during a call utilize an erroneous point in time of transmission in its time slot and disturb other mobiles using the same carrier frequency. To avoid this, the standard allows for a total resynchronization. If the mobile causes large disturbances, one alternative is to disrupt the call. It has been mentioned in the description above that the symbol sequences are delayed and that the mobile is ordered to advance its time of transmission. It may of course also happen that the symbol sequences from the mobile have already been shifted in time in relation to the reference time by a number of symbol times. The mobile may then move so that the symbol sequences arrive too early at the base station. The base station will then order the mobile to reduce its advancement of the time of transmission.
To measure the point in time of transmission of the mobile at the base station has proved to be relatively difficult, due to the fact that the measurement is disturbed by noise. The standardized procedure, to order the mobile to shift its time of transmission by only one symbol time has the advantage that major shifts back and forth in time of the time of transmission are avoided. Such shifts may occur when a mobile mistakes a noise signal for an adjustment order, ordering the mobile to shift its time of transmission by several symbol times. The procedure has, however, the disadvantage that the base station may start issuing orders to the mobile to shift the time of transmission by an increasing number of symbol times, as described above, without these orders being carried out.
The problem of synchronization is treated in the U.S. Pat. No. 4,694,473, which describes synchronization in a mobile radio system for data communication. All transmitted symbol sequences are provided with three different synchronization words. By means of these, the starting point in time for the symbol sequence can be calculated, even if one of these synchronization words is lost due to difficult transmission conditions such as fading.
Synchronization, when setting up a connection, is shown in the U.S. Pat. No. 5,125,102. On a control channel, used for setting up a connection, a synchronization word is repeated three times, and the receiver repeats an acknowledgment word three times during the continued connection procedure.
The U.S. Pat. No. 5,235,615 shows a way to establish synchronous CDMA communication between a base station and a plurality of mobiles. The base station measures the time delay between the different mobiles and the base station itself, by periodically emitting an access signal, measuring the time elapsed when an answer from a specific mobile is received. The delay thus measured is then transferred to the mobile, which performs a synchronization by adjusting its transmission in relation to the periodically issued access signal.
The U.S. Pat. No. 5,229,996 shows a method of achieving synchronization in a TDMA system by adjusting the time of transmission in the time slots from the mobiles of the system. The time adjustment is made by the base station issuing an adjustment command to the mobile. To prevent the detection of a false signal from the mobile, two time windows in the base station are defined, one that corresponds to the time position of the signal before the adjustment order and one that corresponds to the time position where the signal is supposed to be located after the adjustment order.